This study provides a first attempt from a process dynamic perspective to put forward, in general terms and explanations, threshold criteria of dye-laden wastewater treatment using immobilized cell systems (ICSs). Porites corals were selected as packing matrices of ICSs in demonstration of immobilized cell treatment for dye decolorization. With a staircase ramp input of dye in the inlet stream, significant losses of biodecolorization activity were gradually emerged when dye concentrations went over critical loadings for treatment. Design equations also disclosed that the decay rate constant decreased from infinity to a finite value directly caused an operation failure in ICS. According to mean value theorem, the proposed threshold criteria (e.g., maximal allowable treatment capacity) in ICSs could be determined by the method of a constant-slope isocline family, directly indicating treatment limits at various nutrient media in biostimulation. (c) 2006 Elsevier Ltd. All rights reserved.